The aim of this project is to use monoclonal antibodies to isolate purified populations of erythroid progenitor cells and to study the regulation of proliferation and maturation of these isolated progenitors in vitro. I have already characterized a panel of monoclonal antibodies reactive with committed hematopoietic stem cells and their progeny. Using these antibodies it has been possible to selectively isolate late erythroid progenitors (CFU-E) in moderately enriched state. For the specific purposes of this project I will continue to extensively characterize the reactivity with committed and multipotential hematopoietic stem cells of monoclonal antibodies against myeloid associated antigens. These and other monoclonal antibodies will be used to develop strategies to isolate erythroid progenitors in highly enriched populations. The requirement for erythropoietin and other sources of erythroid stimulating activities for the optimal in vitro proliferation and maturation of isolated progenitor cells will be determined, as well as the roles of monocytes and T lymphocytes in modulating growth of isolated progenitor cells. The results of these studies should clarify the roles of specific regulators in in vitro erythropoiesis, and may lead to a better understanding of regulation of normal hematopoiesis in humans.